Thermography

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This article is about the infrared imaging technique. For the printing technique called thermography, see thermographic printing
Image of a small dog taken in mid-infrared ("thermal") light (false color)
Image of a small dog taken in mid-infrared ("thermal") light (false color)
Thermographic image of two ostriches
Thermographic image of two ostriches
Thermographic image of a snake held by a human
Thermographic image of a snake held by a human
Thermographic image of a lion
Thermographic image of a lion
Thermographic image of a traditional building in the background and a 'passive house' in the foreground
Thermographic image of a traditional building in the background and a 'passive house' in the foreground

Infrared Thermography, thermal imaging, or thermal video, is a type of infrared imaging science. Thermographic cameras detect radiation in the infrared range of the electromagnetic spectrum (roughly 900–14,000 nanometers or 0.9–14 µm) and produce images of that radiation. Since infrared radiation is emitted by all objects based on their temperatures, according to the black body radiation law, thermography makes it possible to "see" one's environment with or without visible illumination. The amount of radiation emitted by an object increases with temperature, therefore thermography allows one to see variations in temperature (hence the name). When viewed by thermographic camera, warm objects stand out well against cooler backgrounds; humans and other warm-blooded animals become easily visible against the environment, day or night. As a result, thermography's extensive use can historically be ascribed to the military and security services.

Thermal imaging photography finds many other uses. For example, firefighters use it to see through smoke, find persons, and localize the base of a fire. With thermal imaging, power lines maintenance technicians locate overheating joints and parts, a telltale sign of their failure, to eliminate potential hazards. Where thermal insulation becomes faulty, building construction technicians can see heat leaks to improve the efficiencies of cooling or heating air-conditioning. Thermal imaging cameras are also installed in some luxury cars to aid the driver, the first being the 2000 Cadillac DeVille. Some physiological activities, particularly responses, in human beings and other warm-blooded animals can also be monitored with thermographic imaging.[1]

The appearance and operation of a modern thermographic camera is often similar to a camcorder. Enabling the user to see in the infrared spectrum is a function so useful that ability to record their output is often optional. A recording module is therefore not always built-in.

Instead of CCD sensors, most thermal imaging cameras use CMOS focal plane array(FPA). The most common types are InSb, InGaAs, HgCdTe and QWIP FPA. The newest technologies are using low cost and uncooled microbolometers FPA sensors. Their resolution is considerably lower than of optical cameras, mostly 160x120 or 320x240 pixels, up to 640x512 for the most expensive models. Thermographic cameras are much more expensive than their visible-spectrum counterparts, and higher-end models are often export-restricted. Older bolometers or more sensitive models as InSb require cryogenic cooling, usually by a miniature Stirling cycle refrigerator or liquid nitrogen.

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[edit] Difference between IR film and thermography

IR film is sensitive radiation equivalent to temperatures between 250 °C and 500 °C while the range of thermography is approximately -50 °C to over 2,000 °C. So for a IR film to show something it must be over 250 °C or be reflecting infrared radiation from something that is at least that hot. Night vision goggles normally just amplify the small amount of light that is available outside like starlight or moon light and can't see heat or work in complete darkness.

[edit] Passive vs active thermography

All objects above the absolute zero temperature will emit infrared radiation. Hence, an excellent way to measure thermal variations is to use an infrared vision device, usually a focal plane array (FPA) infrared camera capable of detecting radiation in the mid (3to 5 μm) or long (8 to 14 μm) wave infrared bands, denoted as MWIR and LWIR, respectively, corresponding to two of the high transmittance infrared windows. Abnormal temperature profiles at the surface of an object are an indication of a potential problem.[2]

In passive thermography, the features of interest are naturally at a higher or lower temperature than the background. Passive thermography has many applications such as surveillance of people on a scene, and medical. In active thermography on the other hand, an energy source is required to produce a thermal contrast between the feature of interest and the background. The active approach is necessary in many cases given that the inspected parts are usually in equilibrium with the surroundings.

[edit] Advantages of Thermography

  • It shows a visual picture so that can help compare temperatures over a large area
  • It is capable of catching moving targets in real time
  • Able to find deteriorating components prior to failure
  • Measurement in areas inaccessible or hazardous for other methods
  • It is a non-destructive test method
  • Make easier to find defects in shafts and other metal parts

[edit] Limitations and disadvantages of thermography

 This section may contain original research or unverified claims.
Please improve the article by adding references. See the talk page for details. (April 2008)
  • Quality cameras are expensive (oftn $6,000 USD or above) and are easily damaged[citation needed]
  • Images can be hard to interpret accurately even with experience[citation needed]
  • Accurate temperature measurements are very hard to make because of emissivities[citation needed]
  • Most cameras have ±2% or worse accuracy (not as accurate as contact)[citation needed]
  • Training and staying proficient in IR scanning is time consuming[citation needed]
  • Ability to only measure surface areas

[edit] Applications

Thermal infrared imagers convert the energy in the infrared wavelength into a visible light video display. All objects above 0 kelvins emit thermal infrared energy so thermal imagers can passively see all objects regardless of ambient light. However, most thermal imagers only see objects warmer than -50 °C.

The spectrum and amount of thermal radiation depend strongly on an object's surface temperature. This makes it possible for a thermal camera to display an object's temperature. However, other factors also influence the radiation, which limits the accuracy of this technique. For example, the radiation depends not only on the temperature of the object, but is also a function of the emissivity of the object. Also, radiation also originates from the surroundings and is reflected in the object, and the radiation from the object and the reflected radiation will also be influenced by the absorption of the atmosphere.

[edit] See also

[edit] References

  1. ^ Thermal imaging lights up darkened highways
  2. ^ Maldague X. P. V., Jones T. S., Kaplan H., Marinetti S. and Prystay M. “Chapter 2: Fundamentals of Infrared and Thermal Testing: Part 1. Principles of Infrared and Thermal Testing,” in Nondestructive Handbook, Infrared and Thermal Testing, Volume 3, X. Maldague technical ed., P. O. Moore ed., 3rd edition, Columbus, Ohio, ASNT Press, 2001, 718 p.

[edit] External links

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[edit] History of thermal imager manufacturers